In optics, axial chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same focal plane. It occurs because lenses have a different refractive index for different wavelengths of light (i.e. the dispersion of the lens). The refractive index usually decreases with increasing wavelength. Since the focal length of a lens is dependent on the refractive index, different wavelengths of light will be focused on different focal planes. FIG. 1 demonstrates axial chromatic aberration on a typical lens. The side effect of axial chromatic aberration can be seen as a chromatic halo (of false color, typically blue or purple) around objects in the image. This is due to the fact that camera designers usually don't place the sensor in the focal plane of the blue channel, and therefore it will usually be out of focus. However, other channels might be blurred as well.
The term “purple fringing” (PF) refers to the occurrence of regions of severe false color around very bright objects in the image. For that reason purple fringing will normally take place around areas in the image were pixel values are saturated, meaning that their values are at the highest possible value within the dynamic range of the sensor. Purple fringing is commonly explained as an aspect of axial chromatic aberration, although there may be other explanations for this phenomenon.
FIG. 2 explains purple fringing as an aspect of the axial chromatic aberration effect. The dynamic range of the sensor is shown as a gray horizontal bar. A high contrast edge from the photographed scene is projected onto the sensor by the lens (left). As a result of the lens's chromatic aberration, the blue component is defocused and blurred (middle). Finally, the sensor clips all values above its dynamic range, resulting in a blue fringing around the edge (right).
It is important to distinguish the purple fringing artifact from the lateral chromatic aberration artifact, in which the red, green and blue channels are translated across the radial direction (without being blurred).
FIG. 3 shows an example of purple fringing in a real image. It can be seen that all high-contrast edges are surrounded by purple “halo” artifact.
Current solutions to purple fringing are divided into two categories: Pre-capture solutions and post-capture solutions. Pre-capture solutions avoid purple fringing by using high-quality lenses featuring low axial chromatic aberration. These steps, though capable of reducing purple fringing, cannot promise total elimination of the problem. On the other hand, they require special expensive optical equipment.
Real-time post-capture solutions involve applying various filters on the image, such as low pass filters and non-linear filters. Other post-capture approaches are detailed below. Most of these solutions suffer from a lot of artifacts (such as loss of details, chroma blur, undesired chroma suppression) and are incapable of handling wide purple fringing. There are also manual post-capture solutions which are normally time-consuming and unsuitable for digital still or video camera applications.